1. Field of the Invention
The present invention relates to a method for preventing a head collision in a magnetic storage device, and more particularly, to a method for preventing a head from colliding with a peripheral mechanical unit when the detection of servo synchronizing signals of a constant number fails.
2. Description of the Related Art
Magnetic storage devices, such as a hard disk drive, a floppy disk drive, etc., have widely been used as an auxiliary storage device of a computer system. The hard disk drive not only stores large amount of data stably but accesses the data at high speed. A hard disk drive has a disk which rotates at high speed by a spindle motor and an actuator arm to which a magnetic head (hereinafter, referred to as the head) for writing or reading data in or from a track of the disk is installed. The actuator arm turns round a pivot axis. A bobbin and a coil installed at one end of the actuator arm move by the operation of a voice coil motor. Then the head, installed on an end of a suspension of the other end of the actuator arm, moves horizontally above the disk, thereby writing or reading data in or from the track. In this case, the head moves while maintaining a minute flying height in response to an air flow generated by the high speed revolution of the disk.
To read and write data, servo control is needed for shifting the head to a target track and following up a center line of the target track within a constant range. Servo information for this servo control is written into the disk by a servo writer while the hard disk drive is fabricated. A method for writing the servo information by using the whole of one surface of the disk is called a dedicated servo method. A method for writing the data and servo information on the same disk surface is called an embedded servo method. Use of both the dedicated and embedded servo methods is discussed in U.S. Pat. No. 5,109,307 to Michael Sidman entitled Continuous-Plus-Embedded Servo Data Position Control System For Magnetic Disk Device incorporated herein by reference.
In a system having a disk in which the servo information is written only by the embedded servo method, a servo sector and a data sector are alternatively arranged. The data sector is classified into an identification (ID) area for writing ID information and a data area for writing user data. If a headerless format is utilized, there is no ID area. The servo sector is divided into an automatic gain control (AGC) area, a servo synchronizing signal (SYNC) area, a servo address mark (SAM) area, an index (IDX) area, a gray code area, a servo burst area, and a postamble (PAD) area. The AGC area constantly maintains, throughout the entire disk, time needed to change a data write operation state to a servo information read operation state and the size of a position signal read from the head. The SYNC area provides the synchronization of servo signal detection. In the servo address mark area, a reference pattern of servo timing generation is written. The index area provides one-rotation information of the disk. In the gray code area, address information such as a servo sector, a head, a cylinder number, etc. is written. The servo burst area controls the on-track position of the head.
During track search, in order to accurately read the servo information from the servo sector with the above construction, a servo synchronizing signal should be detected. If the servo synchronizing signal is not normally detected due to a defect of the disk, a damage of a servo synchronizing signal pattern, noise etc., the position information of the head can not be accurately obtained and normal servo control is not performed. Then the hard disk drive counts the number of undetected servo synchronizing signals. If the counted number exceeds a set value, the head is parked to a parking zone.
In a track search mode, a microprocessor for performing the servo control checks whether the servo synchronizing signal is detected. If the servo synchronizing signal is detected, a position error signal (PES) value and a state variable are calculated by using track information and a servo burst signal read through the head. A control output value for servo-controlling the head is supplied to a VCM driver. The microprocessor checks whether a current servo control mode is a settling mode. If yes, the microprocessor checks whether the current servo control mode is an on-track mode. If it is the on-track mode, the number of the undetected servo synchronizing signals is cleared. If the current servo control mode is not the settling mode or if it is the settling mode but not the on-track mode, the microprocessor continuously checks whether the servo synchronizing signal written in the servo sector is detected. If the servo synchronizing signal is not detected, the number of the undetected servo synchronizing signals is counted. The microprocessor checks whether the number of undetected servo synchronizing signals exceeds a set value. If so, the microprocessor sets a servo synchronizing signal detection error bit (hereinafter, referred to as the fail synchronizing bit F-SYNC) and the head is shifted to the parking zone. If the fail synchronizing bit F-SYNC is set while the head is accelerated at high speed, the head is parked under the state that the moving speed of the head is not sufficiently decelerated. Therefore, the head collides with a peripheral mechanical unit such as a crash stopper. Namely, the conventional hard disk drive has a problem in that the head collides with a peripheral mechanical unit if the servo synchronizing signals are not successively detected during the track search.
U.S. Pat. No. 5,355,260 to Masako Mikada entitled Servo System For Positioning A Read/Write Head Within A Storage Device With Means For Detecting Malfunctions In The Servo Data contemplates another servo malfunction process in a hard disk drive that stops the operation of the hard disk drive upon detection of a malfunction, however, the Mikada patent does not describe a process of moving the head to a parking area when the operation is stopped.